Use of conjugate focal plane to generate target information in a lidar system

ABSTRACT

A light detection and ranging (LIDAR) system includes an optical source to emit an optical beam, where a local oscillator (LO) signal is generated from a partial reflection of the optical beam from a partially-reflecting surface proximate to the first focal plane, and where a transmitted portion of the optical beam is directed toward a scanned target environment. LIDAR system to focus the LO signal and a target return signal at a second focal plane comprising a conjugate focal plane to the first focal plane. The system may also include a photodetector with a photosensitive surface proximate to the conjugate focal plane to mix the LO signal with the target return signal to generate target information.

CROSS REFERENCE TO RELATED APPLICATION

The application is a continuation of U.S. patent application Ser. No.17/031,515, filed Sep. 24, 2020, the entire contents of which are herebyincorporated by reference.

FIELD

The present disclosure is related to light detection and ranging (LIDAR)systems in general, and more particularly to the generation of a coaxiallocal oscillator (LO) signal at a conjugate focal plane.

BACKGROUND

Frequency-Modulated Continuous-Wave (FMCW) LIDAR systems use tunablelasers for frequency-chirped illumination of targets, and coherentreceivers for detection of backscattered or reflected light from thetargets that are combined with a local copy of the transmitted signal(LO signal). Mixing the LO signal with the return signal, delayed by theround-trip time to the target and back, generates a beat frequency atthe receiver that is proportional to the distance to each target in thefield of view of the system.

These LIDAR systems employ optical scanners with high-speed mirrors toscan a field of view (FOV) and to de-scan target return signals from theFOV. As mirror speeds are increased, mirror movement during the roundtrip time to and from a target can cause spatial misalignment betweenthe LO signal and the target return signal, in turn, reduces the spatialmixing efficiency in the photodetectors that are used to mix thesignals.

SUMMARY

The present disclosure describes various examples of LIDAR systems andmethods for generating coaxial LO and target return signals for improvedspatial mixing efficiency.

In one example, a LIDAR system according to the present disclosureincludes an optical source to emit an optical beam one or more opticalcomponents coupled with the optical source to focus the optical beam ata first focal plane. A local oscillator (LO) signal is generated from apartial reflection of the optical beam from a partially-reflectingsurface proximate to the first focal plane, and a transmitted portion ofthe optical beam is directed toward a scanned target environment. Theone or more optical components focus the LO signal and a target returnsignal at a second focal plane, which is a conjugate focal plane to thefirst focal plane. The system also includes a photodetector with aphotosensitive surface proximate to the conjugate focal plane to mix theLO signal with the target return signal to generate target information.

In one example, the one or more optical components may be free-spaceoptics that include a polarization beam splitter (PBS) to transmit theoptical beam to a first lens system, where the first lens system focusesthe optical beam at the first focal plane. The free-space optics alsoinclude an optical window containing the partially reflecting surfacewhere the LO signal is generated from the optical beam and reflectedback through the first lens system.

In one example, the free-space optics include a second lens system,where the LO signal is directed through the second lens system by thePBS, and where the second lens system focuses the LO signal and thetarget return signal at the second focal plane.

In one example, the free-space optics include a third lens system tocollimate the transmitted portion of the optical beam and an opticalscanner to scan the target environment with the transmitted portion ofthe optical beam and to de-scan the target return signal. The third lenssystem focuses the target return signal at the first focal plane, thefirst lens system collimates the LO signal and the target return signal,and the PBS directs the LO signal and the target return signal to thesecond lens system.

In one example, the partially reflecting surface is displaced from thefirst focal plane.

In one example, the photodetector is displaced from the second focalplane.

In one example, a method in a LIDAR system according to the presentdisclosure includes focusing an optical beam at a first focal plane;generating a local oscillator (LO) signal by reflecting a portion of theoptical beam from a partially reflecting surface proximate to the firstfocal plane, where a transmitted portion of the optical beam is directedtoward a scanned target environment; focusing the LO signal and a targetreturn signal at a second focal plane conjugate to the first focalplane; and mixing the LO signal with the target return signal in aphotodetector proximate to the second focal plane to generate targetinformation.

In one example, the method includes generating the optical beam with acoherent optical source; transmitting the optical beam through apolarization beam splitter (PBS) and through a first lens system tofocus the optical beam at the first focal plane and through an opticalwindow containing the partially reflecting surface, where the LO signalis reflected back through the first lens system.

In one example, the method includes reflecting the LO signal and thetarget return signal from the PBS through a second lens system, wherethe LO signal and the target return signal are focused at the secondfocal point.

In one example, the method includes collimating the transmitted portionof the optical beam with a third lens system, scanning the targetenvironment with the transmitted portion of the optical beam,de-scanning the target return signal, and focusing the target returnsignal at the first focal plane with the third lens system.

In one example, the method includes collimating the LO signal and thetarget return signal with the first lens system and directing the LOsignal and the target return signal to the second lens system with thePBS, where the LO signal and the target return signal are focused at thesecond focal plane.

In one example of the method, the partially reflecting surface of theoptical window is displaced from the first focal plane.

In one example of the method, the photodetector is displaced from thesecond focal plane.

In one example, a LIDAR system according to the present disclosureincludes a processor and a non-transitory computer-readable mediumstoring instructions, that when executed by the processor, cause theLIDAR system to perform operations, the operations including focusing anoptical beam at a first focal plane; generating a local oscillator (LO)signal by reflecting a portion of the optical beam from a partiallyreflecting surface proximate to the first focal plane, where atransmitted portion of the optical beam is directed toward a scannedtarget environment; focusing the LO signal and a target return signal ata second focal plane conjugate to the first focal plane; and mixing theLO signal with the target return signal in a photodetector proximate tothe second focal plane to generate target information.

In one example, the operations also include generating the optical beamwith a coherent optical source and transmitting the optical beam througha polarization beam splitter (PBS) and through a first lens system tofocus the optical beam at the first focal plane, and through an opticalwindow including the partially reflecting surface, where the LO signalis reflected back through the first lens system.

In one example, the operations also include reflecting the LO signal andthe target return signal from the PBS through a second lens system,where the LO signal and the target return signal are focused at thesecond focal point.

In one example, the operations also include collimating the transmittedportion of the optical beam with a third lens system; scanning thetarget environment with the transmitted portion of the optical beam;de-scanning the target return signal; and focusing the target returnsignal at the first focal plane with the third lens system.

In one example, the operations also include collimating the LO signaland the target return signal with the first lens system and directingthe LO signal and the target return signal to the second lens systemwith the PBS, where the LO signal and the target return signal arefocused at the second focal plane.

In one example, the partially reflecting surface of the optical windowis displaced from the first focal plane.

In one example, the photodetector is displaced from the second focalplane.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the various examples, reference isnow made to the following detailed description taken in connection withthe accompanying drawings in which like identifiers correspond to likeelements:

FIG. 1 illustrates an example FMCW LIDAR system according to embodimentsof the present disclosure;

FIG. 2 is a time-frequency diagram illustrating an example of FMCW LIDARwaveforms according to embodiments of the present disclosure;

FIG. 3 is a block diagram of an example optical system according toembodiments of the present disclosure;

FIG. 4 is a block diagram of an example optical system according toembodiments of the present disclosure;

FIG. 5 is a block diagram of an example optical system according toembodiments of the present disclosure;

FIG. 6 is a block diagram of an example optical system according toembodiments of the present disclosure;

FIG. 7 is a block diagram of an example optical system according toembodiments of the present disclosure;

FIG. 8 is a block diagram of an example optical system according toembodiments of the present disclosure;

FIG. 9 is a block diagram of an example optical system according toembodiments of the present disclosure;

FIG. 10 is a block diagram of an example optical system according toembodiments of the present disclosure;

FIG. 11 is an example of a system of lenses according to embodiments ofthe present disclosure;

FIG. 12 is a block diagram of an example optical system according toembodiments of the present disclosure;

FIG. 13 is a flowchart illustrating an example method for generating acoaxial local oscillator at a conjugate focal plane according toembodiments of the present disclosure;

FIG. 14 is a block diagram of an example system for generating a coaxiallocal oscillator at a conjugate focal plane according to embodiments ofthe present disclosure;

FIG. 15 is a block diagram of an example multi-beam optical systemaccording to embodiments of the present disclosure;

FIG. 16 is a block diagram of an example multi-beam optical systemaccording to embodiments of the present disclosure; and

FIG. 17 is a block diagram of an example multi-beam optical systemaccording to embodiments of the present disclosure.

DETAILED DESCRIPTION

The present disclosure describes various examples of LIDAR systems andmethods for detecting and mitigating the effects of obstructions onLIDAR windows. According to some embodiments, the described LIDAR systemmay be implemented in any sensing market, such as, but not limited to,transportation, manufacturing, metrology, medical, and security systems.According to some embodiments, the described LIDAR system is implementedas part of a front-end of frequency modulated continuous-wave (FMCW)device that assists with spatial awareness for automated driver assistsystems, or self-driving vehicles.

FIG. 1 illustrates a LIDAR system 100 according to exampleimplementations of the present disclosure. The LIDAR system 100 includesone or more of each of a number of components, but may include fewer oradditional components than shown in FIG. 1. As shown, the LIDAR system100 includes optical circuits 101 implemented on a photonics chip. Theoptical circuits 101 may include a combination of active opticalcomponents and passive optical components. Active optical components maygenerate, amplify, and/or detect optical signals and the like. In someexamples, the active optical component includes optical beams atdifferent wavelengths, and includes one or more optical amplifiers, oneor more optical detectors, or the like.

Free space optics 115 may include one or more optical waveguides tocarry optical signals, and route and manipulate optical signals toappropriate input/output ports of the active optical circuit. The freespace optics 115 may also include one or more optical components such astaps, wavelength division multiplexers (WDM), splitters/combiners,polarization beam splitters (PBS), collimators, couplers or the like. Insome examples, the free space optics 115 may include components totransform the polarization state and direct received polarized light tooptical detectors using a PBS, for example. The free space optics 115may further include a diffractive element to deflect optical beamshaving different frequencies at different angles along an axis (e.g., afast-axis).

In some examples, the LIDAR system 100 includes an optical scanner 102that includes one or more scanning mirrors that are rotatable along anaxis (e.g., a slow-axis) that is orthogonal or substantially orthogonalto the fast-axis of the diffractive element to steer optical signals toscan an environment according to a scanning pattern. For instance, thescanning mirrors may be rotatable by one or more galvanometers. Theoptical scanner 102 also collects light incident upon any objects in theenvironment into a return optical beam that is returned to the passiveoptical circuit component of the optical circuits 101. For example, thereturn optical beam may be directed to an optical detector by apolarization beam splitter. In addition to the mirrors andgalvanometers, the optical scanner 102 may include components such as aquarter-wave plate, lens, anti-reflective coated window or the like.

To control and support the optical circuits 101 and optical scanner 102,the LIDAR system 100 includes LIDAR control systems 110. The LIDARcontrol systems 110 may include a processing device for the LIDAR system100. In some examples, the processing device may be one or moregeneral-purpose processing devices such as a microprocessor, centralprocessing unit, or the like. More particularly, the processing devicemay be complex instruction set computing (CISC) microprocessor, reducedinstruction set computer (RISC) microprocessor, very long instructionword (VLIW) microprocessor, or processor implementing other instructionsets, or processors implementing a combination of instruction sets. Theprocessing device may also be one or more special-purpose processingdevices such as an application specific integrated circuit (ASIC), afield programmable gate array (FPGA), a digital signal processor (DSP),network processor, or the like.

In some examples, the LIDAR control systems 110 may include a signalprocessing unit 112 such as a digital signal processor (DSP). The LIDARcontrol systems 110 are configured to output digital control signals tocontrol optical drivers 103. In some examples, the digital controlsignals may be converted to analog signals through signal conversionunit 106. For example, the signal conversion unit 106 may include adigital-to-analog converter. The optical drivers 103 may then providedrive signals to active optical components of optical circuits 101 todrive optical sources such as lasers and amplifiers. In some examples,several optical drivers 103 and signal conversion units 106 may beprovided to drive multiple optical sources.

The LIDAR control systems 110 are also configured to output digitalcontrol signals for the optical scanner 102. A motion control system 105may control the galvanometers of the optical scanner 102 based oncontrol signals received from the LIDAR control systems 110. Forexample, a digital-to-analog converter may convert coordinate routinginformation from the LIDAR control systems 110 to signals interpretableby the galvanometers in the optical scanner 102. In some examples, amotion control system 105 may also return information to the LIDARcontrol systems 110 about the position or operation of components of theoptical scanner 102. For example, an analog-to-digital converter may inturn convert information about the galvanometers' position to a signalinterpretable by the LIDAR control systems 110.

The LIDAR control systems 110 are further configured to analyze incomingdigital signals. In this regard, the LIDAR system 100 includes opticalreceivers 104 to measure one or more beams received by optical circuits101. For example, a reference beam receiver may measure the amplitude ofa reference beam from the active optical component, and ananalog-to-digital converter converts signals from the reference receiverto signals interpretable by the LIDAR control systems 110. Targetreceivers measure the optical signal that carries information about therange and velocity of a target in the form of a beat frequency,modulated optical signal. The reflected beam may be mixed with a secondsignal from a local oscillator. The optical receivers 104 may include ahigh-speed analog-to-digital converter to convert signals from thetarget receiver to signals interpretable by the LIDAR control systems110. In some examples, the signals from the optical receivers 104 may besubject to signal conditioning by signal conditioning unit 107 prior toreceipt by the LIDAR control systems 110. For example, the signals fromthe optical receivers 104 may be provided to an operational amplifierfor amplification of the received signals and the amplified signals maybe provided to the LIDAR control systems 110.

In some applications, the LIDAR system 100 may additionally include oneor more imaging devices 108 configured to capture images of theenvironment, a global positioning system 109 configured to provide ageographic location of the system, or other sensor inputs. The LIDARsystem 100 may also include an image processing system 114. The imageprocessing system 114 can be configured to receive the images andgeographic location, and send the images and location or informationrelated thereto to the LIDAR control systems 110 or other systemsconnected to the LIDAR system 100.

In operation according to some examples, the LIDAR system 100 isconfigured to use nondegenerate optical sources to simultaneouslymeasure range and velocity across two dimensions. This capability allowsfor real-time, long range measurements of range, velocity, azimuth, andelevation of the surrounding environment.

In some examples, the scanning process begins with the optical drivers103 and LIDAR control systems 110. The LIDAR control systems 110instruct the optical drivers 103 to independently modulate one or moreoptical beams, and these modulated signals propagate through the passiveoptical circuit to the collimator. The collimator directs the light atthe optical scanning system that scans the environment over apreprogrammed pattern defined by the motion control system 105. Theoptical circuits 101 may also include a polarization wave plate (PWP) totransform the polarization of the light as it leaves the opticalcircuits 101. In some examples, the polarization wave plate may be aquarter-wave plate or a half-wave plate or a non-reciprocal polarizationrotator such as Faraday rotator. A portion of the polarized light mayalso be reflected back to the optical circuits 101. For example, lensingor collimating systems used in LIDAR system 100 may have naturalreflective properties or a reflective coating to reflect a portion ofthe light back to the optical circuits 101.

Optical signals reflected back from the environment pass through theoptical circuits 101 to the receivers. Because the polarization of thelight has been transformed, it may be reflected by a polarization beamsplitter along with the portion of polarized light that was reflectedback to the optical circuits 101. Accordingly, rather than returning tothe same fiber or waveguide as an optical source, the reflected light isreflected to separate optical receivers. These signals interfere withone another and generate a combined signal. Each beam signal thatreturns from the target produces a time-shifted waveform. The temporalphase difference between the two waveforms generates a beat frequencymeasured on the optical receivers (photodetectors). The combined signalcan then be reflected to the optical receivers 104.

The analog signals from the optical receivers 104 are converted todigital signals using ADCs. The digital signals are then sent to theLIDAR control systems 110. A signal processing unit 112 may then receivethe digital signals and interpret them. In some embodiments, the signalprocessing unit 112 also receives position data from the motion controlsystem 105 and galvanometers (not shown) as well as image data from theimage processing system 114. The signal processing unit 112 can thengenerate a 3D point cloud with information about range and velocity ofpoints in the environment as the optical scanner 102 scans additionalpoints. The signal processing unit 112 can also overlay a 3D point clouddata with the image data to determine velocity and distance of objectsin the surrounding area. The system also processes the satellite-basednavigation location data to provide a precise global location.

FIG. 2 is a time-frequency diagram 200 of an FMCW scanning signal 201that can be used by a LIDAR system, such as system 100, to scan a targetenvironment according to some embodiments. In one example, the scanningwaveform 201, labeled as f_(FM)(t), is a sawtooth waveform (sawtooth“chirp”) with a chirp bandwidth Δf_(C) and a chirp period T_(C). Theslope of the sawtooth is given as k=(Δf_(C)/T_(C)). FIG. 2 also depictstarget return signal 202 according to some embodiments. Target returnsignal 202, labeled as f_(FM)(t−Δt), is a time-delayed version of thescanning signal 201, where Δt is the round trip time to and from atarget illuminated by scanning signal 201. The round trip time is givenas Δt=2R/v, where R is the target range and v is the velocity of theoptical beam, which is the speed of light c. The target range, R, cantherefore be calculated as R=c(Δt/2). When the return signal 202 isoptically mixed with the scanning signal, a range dependent differencefrequency (“beat frequency”) Δf_(R)(t) is generated. The beat frequencyΔf_(R)(t) is linearly related to the time delay Δt by the slope of thesawtooth k. That is, Δf_(R)(t)=kΔt. Since the target range R isproportional to Δt, the target range R can be calculated asR=(c/2)(Δf_(R)(t)/k). That is, the range R is linearly related to thebeat frequency Δf_(R)(t). The beat frequency Δf_(R)(t) can be generated,for example, as an analog signal in optical receivers 104 of system 100.The beat frequency can then be digitized by an analog-to-digitalconverter (ADC), for example, in a signal conditioning unit such assignal conditioning unit 107 in LIDAR system 100. The digitized beatfrequency signal can then be digitally processed, for example, in asignal processing unit, such as signal processing unit 112 in system100. It should be noted that the target return signal 202 will, ingeneral, also include a frequency offset (Doppler shift) if the targethas a velocity relative to the LIDAR system 100. The Doppler shift canbe determined separately, and used to correct the frequency of thereturn signal, so the Doppler shift is not shown in FIG. 2 forsimplicity and ease of explanation. It should also be noted that thesampling frequency of the ADC will determine the highest beat frequencythat can be processed by the system without aliasing. In general, thehighest frequency that can be processed is one-half of the samplingfrequency (i.e., the “Nyquist limit”). In one example, and withoutlimitation, if the sampling frequency of the ADC is 1 gigahertz, thenthe highest beat frequency that can be processed without aliasing(Δf_(Rmax)) is 500 megahertz. This limit in turn determines the maximumrange of the system as R_(max)=(c/2)(Δf_(Rmax)/k) which can be adjustedby changing the chirp slope k. In one example, while the data samplesfrom the ADC may be continuous, the subsequent digital processingdescribed below may be partitioned into “time segments” that can beassociated with some periodicity in the LIDAR system 100. In oneexample, and without limitation, a time segment might correspond to apredetermined number of chirp periods T, or a number of full rotationsin azimuth by the optical scanner.

FIG. 3 illustrates a two-dimensional representation of a system ofoptical components 300 according to various aspects of the presentdisclosure. System 300 may include one or more components of opticalcircuits 101, free-space optics 115 and optical scanner 102 in system100 as illustrated in FIG. 1

System 300 includes an optical source 301 that generates a coherentoptical beam 302 with a selected polarization (e.g., s-polarization orp-polarization). As illustrated by section A-A (303), the optical beam302 has an approximately circular or elliptical cross-section. Theoptical source 301 directs the optical beam 302 to a polarization beamsplitter (PBS) 304 that transmits the selected polarization of theoptical beam 302 to a first lens system 305. According to someembodiments, a polarizing wave-plate or a Faraday rotator can be used toalter reflected polarizations from optical window 307. According to someembodiments, optical window 307 includes a wedge glass that isconfigured to eliminate spatial interference between front and backsurface reflections. In some scenarios, optical window 307 can beconfigured based on a scan speed and/or range of interest. In thisfashion, wedge orientation can induce a shift in LO signal on one ormore photodetectors, which can be used to compensate for the overlapmismatch between return signal and LO signal due to descanning lag infast unidirectional scanners such as spinning polygon.

According to some embodiments, a beam splitter (BS) may be used in placeof PBS 304. The first lens system 305 is configured to focus the opticalbeam 302 at a first focal plane 306. Lens system can be a singleaspheric lens or a multi-element lens design which includes anycombination of spherical and aspherical surfaces.

An optical window 307 has a partially-reflecting surface 308 located atthe first focal plane 306. The partially-reflecting surface 308 reflectsa portion of the optical beam 302 back toward the first lens system 305as a local oscillator (LO) signal 309 with an altered polarization. Theoptical window 307 is substantially perpendicular to the primary opticalaxis 310 of the first lens system 305, so the return path of the LOsignal 309 is substantially the same as the forward path of the opticalbeam 302. Although not depicted, the back surface of the optical window307 may include a partially-reflecting surface. Therefore, an additionalLO signal may be reflected from the optical beam 302 at the back surfaceof the optical window 307. In one embodiment, either the LO signal 309from the front surface or the additional LO signal from the back surfacemay be used as the LO signal. However, of the two LO signal, one may bein-focus while the other may be out-of-focus resulting in competing LOsignals and increased shot noise. Therefore, in some embodiments, theoptical window 307 may be a wedge glass such that the LO signalreflected from the back surface of the optical window 307 does notinterfere with the front surface reflection (e.g., LO signal 309) orvice versa.

The LO signal 309 is collimated by the first lens system 305 anddirected to the PBS 304 where the altered polarization of the LO signal309 is reflected by the PBS 304 to a second lens system 311. Asillustrated by section B-B 312, the LO signal 309 has an approximatelycircular or elliptical cross-section, the same or similar to section A-A303. The second lens system 311 focuses the LO signal 309 at a secondfocal plane 313 that is a conjugate focal plane to the first focal plane306. A photodetector 314 with a photosensitive surface located at thesecond focal plane 313 receives the energy of the LO signal 309.Ideally, the LO signal 309 would be focused to a point on the secondfocal plane 313, but practical limitations on the alignment of theoptical components could result in a measure of defocusing asillustrated by the projection 316 of the LO signal 309 onto the surfaceof the detector 315, which has a non-zero diameter. Additional opticalcomponents of system 300, including a third lens system 317 and anoptical scanner 318 are described below.

FIG. 4 illustrates the full path of the optical beam 302 to a targetenvironment 320, in isolation, according to some embodiments. Afterconverging at the first focal plane 306, the portion of the optical beam302 not reflected from the optical window 307 diverges toward the thirdlens system 317. In one example, the third lens system 317 has the samefocal length as the first lens system 305, so that the optical beam 302is collimated by the third lens system 317. As illustrated by sectionC-C 319 in FIG. 4, the collimated optical beam 302 has an approximatelycircular cross-section.

The collimated optical beam 302 is received by the optical scanner 318,which scans the target environment 320 in azimuth and elevationdirections. Objects in the target environment 320 reflect a portion ofthe optical beam 302 as a return signal as illustrated in FIG.5. In FIG.5, a return signal 321, with an altered polarization from the opticalbeam 302, is de-scanned by the optical scanner 318. Ignoring any effectsfrom de-scanning errors in the optical scanner 318 (described in moredetail below), the de-scanned return signal 321 is a collimated beamwith an approximately circular cross-section as illustrated by sectionD-D 322 in FIG. 5, which is substantially parallel to the principaloptical axis 310 of the third lens system 317 and the first lens system305. Accordingly, the return signal 321 converges at the first focalplane 306 and then diverges toward the first lens system 305, where itis re-collimated. The re-collimated return signal 321 is reflected bythe PBS 304, due to its altered polarization, and directed toward thesecond lens system 311.

The second lens system 311 focuses the return signal 321 at the secondfocal plane 313, as described above with respect to the LO signal 309.According to some embodiments, the return signal 321 can be focused to apoint on the second focal plane 313.

FIG. 6 illustrates the operation of system 300 when all of the opticalcomponents are aligned in a manner that minimizes the opticalaberrations. In one example, the optical beam, LO signal and returnsignal (e.g., optical beam 302, LO signal 309, and return signal 321 ofFIGS. 3 and 4) would all be symmetrically aligned. This alignment isillustrated in FIG. 6 through the use of compound beam notation. Forexample, the beam 322 between the first focal plane 306 and the targetenvironment 320 includes both the outgoing optical beam 302 and theincoming return signal 321; the beam 323 between the first focal point306 and the PBS 304 includes the outgoing optical beam 302, the incomingreturn signal (e.g., return signal 321 as depicted in FIG. 5), and theLO signal (e.g., LO signal 309 of FIG. 3); and the beam 324 between thePBS 304 and the photodetector 314 includes both the LO signal (e.g., LOsignal 309 of FIG. 3) and the return signal (e.g., return signal 321 ofFIG. 5). In this example of system 300, the LO signal and return signal(e.g., LO signal 309 and the return signal 321 of FIGS. 3 and 5) wouldbe focused at a point on the second focal plane 313.

FIG. 7 illustrates a system where the optical window 307 has beenmisaligned by the angleϕ. A misalignment may cause the point focus ofthe LO signal 309 to diverge from the point focus of the return signal321 on the second focal plane 313. The spatial mixing efficiency (γ) ofthe system 300 is a function of the overlap integral of the spot size ofthe LO signal 309 and the spot size of the return signal 321, given by:

$\gamma \propto \frac{{{\int{\int_{{- d}{et}}^{\det}{{E_{LO}\left( {x,y} \right)}*{E_{s}\left( {{x - x_{0}},{y - y_{0}}} \right)}{dxdy}}}}}^{2}}{\left. {\int{\int_{{- d}{et}}^{\det}{{{E_{LO}\left( {x,y} \right)}}^{2} \cdot {\int\int_{{- d}{et}}^{\det}}}}} \middle| {E_{s}\left( {{x - x_{0}},{y - y_{0}}} \right)} \right|^{2}}$

where E_(s) and E_(lo) are the return signal and LO signal electricfield profiles on the photodetector, x₀ and y₀ are the displacement ofthe return signal spot relative to the LO signal spot, and det is theradius of detector 314 for a circular detector. If the LO signal spotand the return signal spot do not overlap, as illustrated in FIG. 7, thespatial mixing efficiency will be reduced to zero.

Mixing efficiency can be maximized, even if a misalignment of theoptical window 307 occurs, by locating the surface 315 of thephotodetector 314 in front of the second focal plane 313 (or behind thesecond focal plane 313 to achieve a similar de-focusing effect) asillustrated in FIG. 8, where the projection of the compound beam 324 onthe surface 315 of the photodetector 314 (which includes the coaxial LOsignal 309 and return signal 321) has an increased diameter comparedwith the spot focus of the compound beam 324 in the configuration ofFIG. 6.

FIG. 9 illustrates the effect of repositioning the photodetector 314 infront of the second focal plane 313 where the LO signal 309 and thereturn signal 321 are misaligned. Rather than two non-overlapping focalpoints, the LO signal 309 and the return signal 321 have overlappingareas on the face 315 of the photodetector 314, where spatial mixing canoccur to generate the baseband signal Δf_(R)(t).

Positioning the photodetector 314 in front of the second focal plane 313may also increase the spatial mixing to combat misalignment between theLO signal 309 and the return signal 321 due to de-scanning lag of returnsignal 321 in the optical scanner 318 caused by potential. At angularvelocities greater than approximately 20,000 degrees per second, thetime delay for return signals, from targets at sufficiently long range,is long enough that the scanning mirror in the optical scanner 318 hastime to rotate a non-negligible angle, causing a skew in the angle ofthe return signal 321 that is reflected to the third lens system 317 bythe optical scanner 318.

This skewing effect is illustrated in the example of FIG. 10, where thereturn signal 321 is skewed from the optical beam 302 as shown insection G-G. The skew angle is propagated through the combination of thethird lens system 317 and the first lens system 305 as illustrated inFIG. 11, where the lens systems are represented by equivalent, thindouble-convex lenses. If the return signal 321 enters the third lenssystem 317 as a plane wave at an angle θ with respect to the principaloptical axis 310, then the return signal 321 will converge at the firstfocal plane 306 at a point determined by the focal length of the thirdlens system 317 and the angle 0. Additionally, as noted previously,since the distance between first and third lens system is equal to thesum of their focal lengths, the return signal 321 will be re-collimatedby the first lens system 305 at the angle 0 with respect to theprincipal optical axis 310.

Returning to FIG. 10, the skewed return signal 321 and the LO signal 309are reflected by the PBS 304 and directed to the second lens system 311as overlapping signals as illustrated by section H-H. Both signals arefocused at the second focal plane 313, but at different points. Thenon-skewed LO signal 309 will be focused on the principal optical axisof the second lens system 311, while the skewed return signal 321 willbe focused at a point offset from the principal optical axis, just asthe third lens system 317 focused the return signal 321 on the firstlocal plane 306. However, since the photodetector 314 is located infront of the second focal plane 313, there is substantial overlapbetween the LO signal 309 and the return signal 321 on the face 315 ofthe photodetector 314.

FIG. 12 illustrates the example system 300 with a modification thatgenerates a complete overlap between the LO signal 309 and the returnsignal 321 on the surface 315 of the photodetector 314. The modificationcomprises a displacement of the partially-reflecting surface 308 of theoptical window 307 by moving the optical window away from the firstfocal plane 306 by a distance δ toward the third lens system 317 in theexample of FIG. 12. It should be noted that the same effect can beachieved by moving the optical window away from the focal plane 306 inthe opposite direction, toward the first lens system 305. Thisdisplacement causes the optical beam 302 to be reflected from thepartially-reflecting surface 308 after it has diverged from its focalpoint. As a result, the LO signal 309 reflected from thepartially-reflecting surface 308 has a substantially greater diameterthan it would have if reflected at the focal plane 306, as illustratedby section J-J.

As illustrated in FIG. 12, the second lens system 311 focuses the LOsignal 309 and the return signal 321 toward the second focal plane 313.The location of the photodetector 314 and the increased diameter of theLO signal 309 result in a complete overlap of the LO signal 309 and thereturn signal 321 to maximize the spatial mixing efficiency.

FIG. 13 is a flowchart illustrating an example method 400 in a LIDARsystem for generating a coaxial local oscillator signal at a conjugatefocal plane according to embodiments of the present disclosure. Method400 begins at operation 402, focusing an optical beam (e.g., opticalbeam 302) at a first focal plane (e.g., focal plane 306). Method 400continues at operation 404, generating a local oscillator (LO) signal(e.g., LO signal 309) by reflecting a portion of the optical beam from apartially reflecting surface (e.g., surface 308 on optical window 307)proximate to the first focal plane, where a transmitted portion of theoptical beam is directed toward a scanned target environment (e.g., byoptical scanner 318). Next, method 400 continues at operation 406,focusing the LO signal and a target return signal (e.g., return signal321) at a second focal plane (e.g., second focal plane 313) conjugate tothe first focal plane. Method 400 concludes with operation 408, mixingthe LO signal with the target return signal in a photodetector (e.g.,photodetector 314) proximate to the second focal plane to generatetarget information.

FIG. 14 is a block diagram illustrating an example processing system 500in a LIDAR system for generating a coaxial local oscillator signal at aconjugate focal plane according to embodiments of the presentdisclosure. Processing system 500 includes a processor 501. In oneexample, processor 501 may be embedded in the signal processing unit 112in the LIDAR control systems 110 in LIDAR system 100. In some examples,501 may be one or more general-purpose processing devices such as amicroprocessor, central processing unit, or the like. More particularly,processor 501 may be a complex instruction set computing (CISC)microprocessor, reduced instruction set computer (RISC) microprocessor,very long instruction word (VLIW) microprocessor, or processorimplementing other instruction sets, or processors implementing acombination of instruction sets. The processor 501 may also be one ormore special-purpose processing devices such as an application specificintegrated circuit (ASIC), a field programmable gate array (FPGA), adigital signal processor (DSP), network processor, or the like.

Processing system 500 also includes a computer-readable memory 502coupled to the processor 501. Memory 502 may be, for example, read-onlymemory (ROM), random-access memory (RAM, programmable read-only memory(PROM), erasable programmable read-only memory (EPROM), electricallyerasable programmable read-only memory (EEPROM), flash memory, magneticdisk memory such hard disk drives (HDD), optical disk memory such ascompact-disk read-only (CD-ROM) and compact disk read-write memory(CD-RW), or any other type of non-transitory memory.

In some examples, memory 502 includes instructions that, when executedby the processor 501, cause a LIDAR system (e.g., LIDAR system 100) togenerate a coaxial local oscillator signal (e.g., LO signal 309) at aconjugate focal plane (e.g., second focal plane 313) according toembodiments of the present disclosure.

In one example, memory 502 includes instructions 504 for focusing anoptical beam (e.g., optical beam 302) at a first focal plane (e.g.,focal plane 306); instructions 506 for generating a local oscillator(LO) signal (e.g., LO signal 309) by reflecting a portion of the opticalbeam from a partially reflecting surface (e.g., surface 308 of opticalwindow 307) proximate to the first focal plane, where a transmittedportion of the optical beam is directed toward a scanned targetenvironment (e.g., by optical scanner 318); instructions 508 forfocusing the LO signal and a target return signal (e.g., return signal321) at a second focal plane conjugate to the first focal plane (e.g.,second focal plane 313); and instructions 510 for mixing the LO signalwith the target return signal in a photodetector (e.g., photodetector314) proximate to the second focal plane to generate target information.

FIG. 15 is a block diagram of an example multi-beam LIDAR system 600 forgenerating coaxial local oscillator signals at a conjugate planeaccording to embodiments of the present disclosure. System 600 isfunctionally similar to system 300, which has already been described indetail, except that system 600 includes multiple FMCW optical sources601-1 through 601-n, where each optical source may operate at adifferent frequency and/or bandwidth and emit a corresponding opticalbeams 602-1 through 602-n (collectively, optical beams 602). Each of theoptical beams 602 is passed by a polarization beam splitter (PBS) 604 toa corresponding first lens system 605-1 through 605-n (collectively,first lens systems 605). Each first lens system 605 focuses itscorresponding optical beam 602 to a focal point on a first focal plane606. The optical beams 602 then diverge beyond the first focal plane606, where they are partially reflected by a partially-reflectingsurface 608 on an optical window 607. The reflected portion of eachoptical beam 602 comprises a corresponding local oscillator (LO) signal609-1 through 609-n (collectively, LO signals 609).

The LO signals 609 are each collimated by their respective first lenssystem 605 and then reflected by PBS 604 toward corresponding secondlens system 611-1 through 611-n (collectively, second lens systems 611).Each second lens system 611 focuses its corresponding LO signal 609 to afocal point on a second focal plane 613 that is conjugate to the firstfocal plane 606. Each LO signal 609 is intercepted by a correspondingphotodetector 611-1 through 611-n (collectively, photodetectors 611)such that each photodetector 611 is illuminated by a corresponding LOsignal 609 with a non-zero diameter.

The portion of each optical beam 602 that is not reflected from opticalwindow 606 is collimated by a third lens system 618 and transmitted toan optical scanner 618. Optical scanner 618 scans a target environment620 with the optical beams 602, and de-scans corresponding target returnsignals 621-1 through 621-n (collectively, return signals 621). Thereturn signals 621 are focused by third lens system 617 at the firstfocal plane 606 and then diverge to intercept the corresponding firstlens system 605. Each first lens system 605 collimates its correspondingreturn signal 621, which are then reflected by PBS 604 toward secondlens systems 611. Each second lens system 611 focuses its correspondingreturn signal 621 on the second focal plane 613. The return signals 621illuminate their corresponding photodetector 614, where they overlap andspatially mix with a corresponding LO signal 609.

FIG. 16 is a block diagram of an example multi-beam LIDAR system 700 forgenerating coaxial local oscillator signals at a conjugate planeaccording to embodiments of the present disclosure. System 700 issimilar in all respects to system 600, except that system 700 includesindependent optical windows 607-1 through 607-n, which can be adjustedindependently with respect to their offset from the first focal plane606.

FIG. 17 is a block diagram of an example multi-beam LIDAR system 800 forgenerating coaxial local oscillator signals at a conjugate planeaccording to embodiments of the present disclosure. System 800 issimilar in all respects to system 700, except that system 800 includesindependent third lens systems 617-1 through 617-n.

The preceding description sets forth numerous specific details such asexamples of specific systems, components, methods, and so forth, inorder to provide a thorough understanding of several examples in thepresent disclosure. It will be apparent to one skilled in the art,however, that at least some examples of the present disclosure may bepracticed without these specific details. In other instances, well-knowncomponents or methods are not described in detail or are presented insimple block diagram form in order to avoid unnecessarily obscuring thepresent disclosure. Thus, the specific details set forth are merelyexemplary. Particular examples may vary from these exemplary details andstill be contemplated to be within the scope of the present disclosure.

Any reference throughout this specification to “one example” or “anexample” means that a particular feature, structure, or characteristicdescribed in connection with the examples are included in at least oneexample. Therefore, the appearances of the phrase “in one example” or“in an example” in various places throughout this specification are notnecessarily all referring to the same example.

Although the operations of the methods herein are shown and described ina particular order, the order of the operations of each method may bealtered so that certain operations may be performed in an inverse orderor so that certain operation may be performed, at least in part,concurrently with other operations. Instructions or sub-operations ofdistinct operations may be performed in an intermittent or alternatingmanner.

The above description of illustrated implementations of the invention,including what is described in the Abstract, is not intended to beexhaustive or to limit the invention to the precise forms disclosed.While specific implementations of, and examples for, the invention aredescribed herein for illustrative purposes, various equivalentmodifications are possible within the scope of the invention, as thoseskilled in the relevant art will recognize. The words “example” or“exemplary” are used herein to mean serving as an example, instance, orillustration. Any aspect or design described herein as “example” or“exemplary” is not necessarily to be construed as preferred oradvantageous over other aspects or designs. Rather, use of the words“example” or “exemplary” is intended to present concepts in a concretefashion. As used in this application, the term “or” is intended to meanan inclusive “or” rather than an exclusive “or”. That is, unlessspecified otherwise, or clear from context, “X includes A or B” isintended to mean any of the natural inclusive permutations. That is, ifX includes A; X includes B; or X includes both A and B, then “X includesA or B” is satisfied under any of the foregoing instances. In addition,the articles “a” and “an” as used in this application and the appendedclaims should generally be construed to mean “one or more” unlessspecified otherwise or clear from context to be directed to a singularform. Furthermore, the terms “first,” “second,” “third,” “fourth,” etc.as used herein are meant as labels to distinguish among differentelements and may not necessarily have an ordinal meaning according totheir numerical designation.

What is claimed is:
 1. A light detection and ranging (LIDAR) system,comprising: one or more optical components coupled with the opticalsource to focus an optical beam at a first focal plane, wherein a localoscillator (LO) signal is generated from a partial reflection of theoptical beam from a partially reflecting surface proximate to the firstfocal plane and reflected back through a first lens system to focus theoptical beam at the first focal plane, wherein a transmitted portion ofthe optical beam is directed toward a scanned target environment, theone or more optical components further to focus the LO signal and atarget return signal at a second focal plane comprising a conjugatefocal plane to the first focal plane; and a photodetector comprising aphotosensitive surface proximate to the conjugate focal plane to mix theLO signal with the target return signal to generate target information.2. The system of claim 1, wherein the partially reflecting surface is anoptical window.
 3. The system of claim 2, wherein the free-space opticsfurther comprise a second lens system, wherein the LO signal is directedthrough the second lens system by polarization beam splitter (PBS), thesecond lens system to focus the LO signal and the target return signalat the second focal plane.
 4. The system of claim 3, wherein thefree-space optics further comprise: a third lens system to collimate thetransmitted portion of the optical beam; and an optical scanner coupledwith the third lens system to scan the target environment with thetransmitted portion of the optical beam, and to de-scan the targetreturn signal, the third lens system to focus the target return signalat the first focal plane, the first lens system to collimate the LOsignal and the target return signal, and the PBS to direct the LO signaland the target return signal to the second lens system.
 5. The system ofclaim 2, wherein the partially reflecting surface is displaced from thefirst focal plane.
 6. The system of claim 2, wherein the photodetectoris displaced from the second focal plane.
 7. A method in a lightdetection and ranging (LIDAR) system, comprising: focusing an opticalbeam at a first focal plane; generating a local oscillator (LO) signalby partially reflecting a portion of the optical beam from a partiallyreflecting surface proximate to the first focal plane and reflecting theLO signal back through a first lens system to focus the optical beam atthe first focal plane, wherein a transmitted portion of the optical beamis directed toward a scanned target environment; focusing the LO signaland a target return signal at a second focal plane conjugate to thefirst focal plane; and mixing the LO signal with the target returnsignal in a photodetector proximate to the second focal plane togenerate target information.
 8. The method of claim 7, generating theoptical beam with a coherent optical source.
 9. The method of claim 7,further comprising reflecting the LO signal and the target return signalfrom a polarization beam splitter (PBS) through a second lens system,wherein the LO signal and the target return signal are focused at thesecond focal plane.
 10. The method of claim 9, further comprising:collimating the transmitted portion of the optical beam with a thirdlens system; scanning the target environment with the transmittedportion of the optical beam; de-scanning the target return signal; andfocusing the target return signal at the first focal plane.
 11. Themethod of claim 10, further comprising: collimating the LO signal andthe target return signal with the first lens system; and directing theLO signal and the target return signal to the second lens system withthe PBS, wherein the LO signal and the target return signal are focusedat the second focal plane.
 12. The method of claim 7, wherein thepartially reflecting surface is displaced from the first focal plane.13. The method of claim 7, wherein the photodetector is displaced fromthe second focal plane.
 14. A non-transitory computer-readable mediumstoring instructions, that when executed by the processor of a lightdetection and ranging (LIDAR) system, cause the LIDAR system to: focusan optical beam at a first focal plane; generate a local oscillator (LO)signal by partially reflecting a portion of the optical beam from apartially reflecting surface proximate to the first focal plane andreflecting the LO signal back through a first lens system to focus theoptical beam at the first focal plane, wherein a transmitted portion ofthe optical beam is directed toward a scanned target environment; focusthe LO signal and a target return signal at a second focal planeconjugate to the first focal plane; and mix the LO signal with thetarget return signal in a photodetector proximate to the second focalplane to generate target information.
 15. The non-transitorycomputer-readable medium of claim 14, wherein the LIDAR system furtherto generate the optical beam with a coherent optical source.
 16. Thenon-transitory computer-readable medium of claim 14, wherein the LIDARsystem further to reflect the LO signal and the target return signalfrom a polarization beam splitter (PBS) through a second lens system,wherein the LO signal and the target return signal are focused at thesecond focal plane.
 17. The non-transitory computer-readable medium ofclaim 16, wherein the LIDAR system further to: collimate the transmittedportion of the optical beam with a third lens system; scan the targetenvironment with the transmitted portion of the optical beam; de-scanthe target return signal; and focus the target return signal at thefirst focal plane.
 18. The non-transitory computer-readable medium ofclaim 17, wherein the LIDAR system further to: collimate the LO signaland the target return signal with the first lens system; and direct theLO signal and the target return signal to the second lens system withthe PBS, wherein the LO signal and the target return signal are focusedat the second focal plane.
 19. The non-transitory computer-readablemedium of claim 14, wherein the partially reflecting surface isdisplaced from the first focal plane.
 20. The non-transitorycomputer-readable medium of claim 14, wherein the photodetector isdisplaced from the second focal plane.